July 28, 2000
01:00 AM (EDT)

News Release Number: STScI-2000-26

Hubble Sees Comet Linear Blow its Top

July 28, 2000: Lackluster comet LINEAR (C/1999 S4) unexpectedly threw astronomers a curve. Using
the Hubble telescope, researchers were surprised to catch the icy comet in a brief,
violent outburst when it blew off a piece of its crust, like a cork popping off
a champagne bottle. The eruption, the comet's equivalent of a volcanic explosion
(though temperatures are far below freezing, at about minus 100 degrees Fahrenheit
in the icy regions of the nucleus or core), spewed a great deal of dust into space.
This mist of dust reflected sunlight, dramatically increasing the comet's brightness
over several hours. Hubble's sharp vision recorded the entire event and even snapped
a picture of the chunk of material jettisoned from the nucleus and floating away
along the comet's tail.

Q & A: Understanding the Discovery

1.
What do the pictures show?

The orbiting observatory's Space Telescope Imaging Spectrograph tracked the streaking comet for two days, July 5 to 7, capturing a dramatic leap in its brightness [left image]; followed by seeing a wave of newly created dust from the outburst flowing into the coma, a shell of dust surrounding the core [middle image]; and culminating in the discovery of a castoff chunk of material from the nucleus sailing along its tail [the bright dot trailing behind the comet in the picture at right]. The white region represents the brightest part of the coma. The nucleus, which is only about a mile wide, cannot be seen in these images because it's too small for the Hubble telescope to see.

2.
What caused the eruption?

Astronomers list several theories for the eruption. One possible reason is that a particularly volatile region of the core became exposed to sunlight for the first time and vaporized away very suddenly. Another possibility is that a buildup of gas pressure from sublimating ice (a change from ice to gas) trapped just below the comet's surface explosively "blew the lid off" a pancake-shaped layer of crust from its surface. The pressure from sunlight blew the fragment down the tail  much like the wind propels a sailboat  where it disintegrated into smaller and smaller pieces, eventually becoming too small to see.

Yet another possibility is that the observed fragment is one of the house-sized "cometesimals" that are thought to make up the nucleus. Evidence accumulated during the past decade suggests that comet nuclei are "rubble piles" of loosely held together cometesimals. Perhaps one of the "building blocks" comprising the core broke off and was blown down the tail by a gaseous jet shooting off the comet's surface like a garden hose spray.